No abstract

A final-state wavefunction, involving three appropriate confluent hypergeometric functions, which satisfies the correct asymptotic condition is used to study positron-impact ionization of atomic hydrogen Results are reported for the double as well as triple difterenlial cross sections and compared with lhe available theoretical findings. The double difterential cross section curves for diKerent ejection angles show the presence of a cusp-like structure at the region of equal energy sharing by the outgoing positron and lhe ejected electron. This is in conformity with the findings of Mandal et a1 at zero ejection angle but unlike the structure exhibited by the curves of Schdtz and Reinhold.

The distorted-wave approximation is applied to investigate the rearrangement collision problem of positronium formation in the ground state with positron scattering from hydrogen and helium atoms for the positron energy ranges 8.704-200 eV and 20-100 eV respectively.

The continuum distorted-wave approximation of Cheshire (1964) is applied to the case in which a nucleus of charge Z' captures an electron attached to another nucleus of charge Z. Numerical results for the total capture cross sections have been obtained for the particular cases of electron capture from the targets He+, Li2+, Be3+ and C5+ by protons into the ground state or the excited 2s state. Capture by alpha particles from He+, Li2+ and Be3+ into the ground state or the excited 2s state are also considered. Wherever possible comparisons are made with the Coulomb-Born results.

Depending on the distribution of energy between the scattered positron and the ejected electron in the final channel, two competing processes are involved in positron-impact ionization of atoms, namely (a) direct head-on ionization of an electron and (b) positronium formation to the continuum. Using Faddeev's three-body scattering formalism for positron-hydrogen-atom collisions, we get an amplitude which takes account of both these processes. The doubly differential cross section thus obtained for the forward scattering angle shows a cusp when plotted against the emission energy. This cusp is due to the effect of positronium formation to the continuum and heretofore has not been predicted.Two interesting competing processes are involved when an energetic positron ionizes an atom or a molecule. These are (a) direct ionization, whereby the emitted electron moves in the continuum relative to the residual ion, and (b) the electron capture (or positronium formation) to the continuum. In describing positron-impact ionization they cannot be separated from one another. That is to say, one does not really know for certain to which of the two centers -the residual ion or the scattered positron -the continuum electron is attached. When the ejected electron moves slowly away from the ion while the positron scatters away much faster, we may say that the electron is in an eigenstate of the residual ion and that direct ionization has taken place.If the velocity of the outgoing electron is small relative to the positron, with their center of mass moving much faster away from the ion, the final state is more an eigenstate of the electron-positron pair, and positronium formation to the continuum should be the dominant mechanism for ionization. As a result, the actual amplitude for positron-impact ionization should be a proper combination of these two physical possibilities. The ionization cross section is therefore not given by the algebraic sum of the individua1 cross sections. In this Brief Report, we consider the specific case of positron-hydrogen-atom collisions and use the threebody scattering formalism of Faddeev' to construct the final-state wave function for an ionizing collision. One essential feature of this approach is that all the interacting particles are treated equally without preference to any one particular pair. The total scattering amplitude for positronimpact ionization thus comes out naturally as an appropriate combination of the amplitude for direct ionization of an electron and the amplitude for positronium formation to the continuum, along with a component which describes the situation where all the interacting particles are asymptotically free. Our findings predict that when the available energy in the final channel is shared almost equally by the outgoing positron and the emitted electron, ionization occurs predominantly through the positronium formation to the continuum.In this case thc triply differential cross section is highly enhanced in the forward direction and the doubly differential cross section...